1. Field of the Invention
The present invention relates to an image forming apparatus and a method of controlling the image forming apparatus.
2. Description of Related Art
In recent years, belt-transfer type image forming apparatuses are known. In the belt-transfer type image forming apparatuses, a transfer belt travels in contact with a photoconductor drum, and a recording sheet is conveyed in synchronization with a toner image formed on the photoconductor drum. A transfer voltage having a polarity opposite to a charging polarity of toner (transfer polarity) is applied to the transfer belt to transfer the toner image on the photoconductor drum onto the recording sheet by electrostatic attraction.
However, belt-transfer type image forming apparatuses have the following problems. Specifically, when the transfer belt is left in a high humidity environment for long periods of time, the transfer belt absorbs moisture in the air, and the electric resistance thereof decreases. FIG. 7 is a graph of variation in surface resistance of a transfer belt versus leaving time under a high humidity environment (30[° C.] and 80[%]). Curve L0 represents variations in surface resistance of a new transfer belt, and curve L1 represents variations in surface resistance of a transfer belt which has been used for printing two million sheets. In both transfer belts, the longer the leaving time under a high humidity environment, the lower the resistance value thereof.
When the surface resistance of a transfer belt decreases, the transfer electric charge of the front-end of a recording sheet diffuses, and thus the adsorption force between the transfer belt and the front-end of the recording sheet is reduced and a performance of separating recording sheets from the photoconductor drum is considerably degraded. In addition, when the use history (for example, the total number of sheets printed by an image forming apparatus) of a transfer belt increases, the performance of separating a recording sheet tends to be further degraded since discharge products and the like are formed on the inner periphery surface of the transfer belt and thus the hygroscopic property of the transfer belt is increased.
Further, the resistance value of the transfer belt and the photoconductor drum, and a transfer electric field that acts on the front-end portion of a recording sheet are varied by many parameters such as humidity, the leaving time of the transfer belt, the use history of the transfer belt, and the number of outputs since the start of printing. Therefore, it is difficult to control the performance of separating recording sheets from the photoconductor drum under a high humidity environment.
FIG. 8 is a graph of variation in separation failure rate (which is referred to also as “separation jam rate”) versus leaving time under a high humidity environment (30[° C.] and 80[%]). Curve L2 illustrates a variation in separation failure rate in the case where a new transfer belt is used, and curve L3 illustrates a variation in separation failure rate in the case where a transfer belt which has been used for printing two million sheets is used. In both cases, as the leaving time under a high humidity environment is prolonged, the separation failure rate is increased.
Japanese Patent Application Laid-Open No. 2003-57966 discloses a technique in which the timing of front-end transfer current switching and an inter-paper transfer current value are brought under switching control in accordance with the resistance value of transfer conveyance means (transfer belt), to thereby improve the performance of separating the transfer sheet from a photoconductor. According to the technique disclosed in Japanese Patent Application Laid-Open No. 2003-57966, when the resistance value of the transfer belt is small, the timing of the front-end transfer current switching is delayed, and at the same time the inter-paper transfer current value is decreased.
In the technique disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 2003-57966, the adsorption force between a transfer sheet and a photoconductor may possibly be decreased, without supplying electric charges to the transfer sheets more than necessary (without overcharging the transfer sheets). However, the direction of the electric field generated between the transfer sheet and the photoconductor is a direction in which the transfer sheet and the photoconductor attract each other, albeit weakly, and therefore the adsorption force between the transfer sheet and the transfer belt may be undesirably decreased at the same time. Therefore, the effect of improving the performance of separating a sheet from the photoconductor cannot be sufficiently obtained by the technique disclosed in Japanese Patent Application Laid-Open No. 2003-57966.